Conventionally, this type of vehicular light has been disclosed under the name “vehicular light” in Japanese Patent Application Laid-Open No. 2017-183287.
FIG. 1 shows a conventional vehicular light disclosed in the aforementioned laid-open publication. The vehicular light has a flat plate-shaped light guide body 80 having a substantially triangular shape. The light guide body 80 includes end surfaces corresponding to respective sides of the triangle and serving as optical functional surfaces as a light incident portion 81, a light reflection portion 82, and a light exit surface 83.
A plurality of LED light sources 84 are arranged at predetermined intervals in the vicinity of the light incident portion 81. In the light incident portion 81, lens cut portions 85 are formed at corresponding positions on the respective optical axes of the LED light sources 84. The lens cut portion 85 is configured to receive the light emitted from each LED light source 84 while refracting the received light to take it into the light guide body as parallel light.
The light guide body 80 is provided with a plurality of through slits 86 each extending along the parallel light introduced into the light guide body 80 from the LED light source 84 side toward the distal end side at a corresponding intermediate position between the adjacent LED light sources 84.
Light rays emitted from each LED light source 84 are collimated by the lens cut portion 85 and enter the light guide body 80. The light rays having entered are internally (totally) reflected by the light reflection portion 82, and the reflected light rays are allowed to exit to the outside through the partial region 83a of the light exit surface 83 corresponding to the LED light source 84.
In this case, the through slit 86 extending within the light guide body 80 functions to prevent the light rays emitted from each LED light source 84 from exiting through a region other than the partial region 83a corresponding to the LED light source 84. This can be achieved by reflecting (totally reflecting) the light rays of oblique components other than parallel components out of the light rays emitted from the respective LED light sources 84 and introduced into the light guide body 80 to prevent the light rays from the respective LED light sources 84 from being mixed with one another.
At the same time, the through slit 86 has a function of giving a transmission loss to the reflected light internally reflected (totally reflected) by the light reflection portion 82 corresponding to the optical path length (light guide length) of the light rays emitted from each LED light source 84 and guided through the light guide body 80, thereby achieving uniformity of the luminance of the light rays exiting through the respective partial regions 83a. 
Incidentally, the through slit 86 extending in the light guide body 80 is merely a slit (groove) that extends along the parallel light introduced into the light guide body 80 and penetrates the light guide body 80 in the thickness direction, and no effective countermeasure is taken to cover the disadvantage among the optical merits and demerits caused by the presence of the through slit 86. Therefore, it is difficult to say that the configuration of the through slit 86 provided in the light guide body 80 is optically suitable and sufficient.